What is the meaning of vertical bars in paths of high symmetry points? I am a new to the study of high symmetry paths. After looking at the Silicon path that is $Γ—X—U|K—Γ—L—W—X$, I am not able to understand the meaning of $U|K$ in this path? 
 A: Silicon's crystal structure is the diamond crystal structure and the Bravais lattice is the fcc lattice. The first Brillouin-Zone (a truncated octahedron) looks like this:

(Image source and further information)
As you can see, the path includes every one of the red lines, always connecting neighboring symmetry points, except for a discontinuity at $K|U$.
The wikipedia page for "bitruncated cubic honeycomb", which is the space-filling tesselation made up of truncated octahedra also containes an image that shows how these Brillouin-Zones are aligned in 3D $\boldsymbol{k}$-space:

If you compare this image with the one of the first Brillouin-Zone above, you can see that by continuing the reciprocal lattice, every $K$-point overlaps with a $U$-point. This means, that for example the value of every band has to be identical at $K$ and $U$. This is why you will see jumps from $K$ to $U$ in bandstructure diagrams.
In conclusion, the vertical bar in $K|U$ marks that both points are equivalent in $\boldsymbol{k}$-space even though the path has a discontinuity there.
